Optical sensors, such as backside illumination sensors, are used in a variety of camera arrangements and are sometimes used in low light environments. A backside illumination sensor relies on passing light through a wafer layer for sensing incident light. However, conventional techniques of layer thinning form a layer having significant thickness variation across the layer. Variations in the layer thickness cause uneven propagation of light through the layer, which in turn causes distortion in the image collected by the optical sensor.
In some arrangements, a chemical and mechanical polishing (CMP) device polishes a wafer layer for a constant polishing time. An etching process removes additional layer material as well as cracks and crystal dislocations introduced during the CMP process to thin the wafer layer to a desired thickness. As the etching duration increases beyond the time needed to remove the cracks and dislocations, the etching process begins to increase non-uniformity in the wafer layer thickness.
A material removal rate for a CMP device is affected by many variables which are different from device to device and between batches of wafers. For example, rotational speed of a platen and a chuck, misalignment between axes of rotation of the platen and the chuck, a force pressing the wafer against the pad, etc. impact the material removal rate of a CMP device. In addition, variations in a concentration of the slurry and wear patterns on the pad impact the material removal rate for each batch of wafers. Some techniques fail to account for these variables and instead use a constant material removal rate. Some techniques, using a constant material removal rate, achieve a total thickness variation of about 0.2 μm on the surface of the wafer layer.